IANAL, but with the existence of Bitcoin futures on CME and CBOE, the CFTC presumably has some jurisdiction over insider trading in BTC spot markets, insomuch as they affect futures prices.

Under the 'Misappropriation Theory', insider trading occurs when confidential information is misappropriated for a trading gain. This interpretation of the law has been used by the CFTC in the past [0] to prosecute insider trading cases.

Did Coinbase employees (or their friends/family) commit IT under this definition? I don't know, but if I was the General Counsel of Coinbase I would be hastily typing up a mandatory internal compliance policy for immediate disclosure of cryptocurrency holdings, pre-clearance of all cryptocurrency trades, and a minimum 30-day holding period to discourage day-trading. Such policies are very standard at traditional financial institutions that have access to material non-public information (MNPI) about the financial markets.

[0] - https://www.cov.com/-/media/files/corporate/publications/201...

If you find yourself asking "Is it illegal to do something not explicitly barred by the law but something that seems dishonest?" then you should be prepared for consequences.

Prosecutors, judges, attorneys and even jurors aren't stupid. There are remarkably broad statutes out there, fraud or theft or something could reasonably be brought to bear upon you.